The other investor who owns 30% is not asking for this. I own 50% equity. I have been taking care of the day to day runnings of the business as CEO and Founder.
That is the problem of taking money in. Investors have the right to "scrutinize" transactions if they want. It is also a sign that probably you are not reporting they way it should be. You should maybe grant access to the investor to you backend via an api or even just allow them to have access to the data base you keep your records. If you do something like stripe or paypal just grant him access as an additional user.